Ovine and Caprine Interleukin-10 (IL-10) is a highly conserved anti-inflammatory cytokine that is identical at the protein level between sheep (Ovis aries) and goats (Capra hircus), reflecting their close phylogenetic relationship. Produced primarily by regulatory T cells (Tregs), Th2 cells, macrophages, dendritic cells, and certain B-cell populations, IL-10 plays a central role in limiting excessive immune activation and maintaining immune homeostasis in small ruminants. IL-10 signals through the IL-10 receptor complex (IL-10R1/IL-10R2), activating JAK/STAT3 pathways that suppress pro-inflammatory cytokine production (including IL-1β, TNF-α, IL-6, and IFN-γ), reduce antigen presentation, and downregulate Th1-type cell-mediated responses. In ovine and caprine health, IL-10 is critically involved in modulating immune responses during economically significant infections such as maedi-visna virus (ovine progressive pneumonia), caprine arthritis encephalitis virus (CAE), bluetongue virus, peste des petits ruminants virus (PPRV), mastitis, parasitic infestations, and bacterial pneumonia, where elevated IL-10 can protect tissues from excessive inflammation but may also contribute to chronic infection and pathogen persistence. The IL-10/pro-inflammatory cytokine balance influences disease severity, vaccine efficacy, and overall herd health. As a biomarker of immune regulation, ovine/caprine IL-10 is valuable in studies of host–pathogen interactions, immune tolerance, and small ruminant vaccine development, supporting comparative immunology research and strategies to improve productivity and disease resilience in sheep and goat populations.